1. Field of the Inventions
The present inventions are related to fuel cells.
2. Description of the Related Art
Fuel cells, which convert reactants (i.e. fuel and oxidant) into electricity and reaction products, are advantageous because they are not hampered by lengthy recharging cycles, as are rechargeable batteries, and are relatively small, lightweight and produce virtually no environmental emissions. Nevertheless, the present inventor has determined that conventional fuel cells are susceptible to improvement in the area of air flow management.
There are a variety of operating requirements associated with the air flow demand in a fuel cell. The oxidant in a fuel cell reaction is frequently the oxygen in ambient air and, accordingly, air flow is required to maintain the reaction. The air flow is also used to transport water vapor, which is a byproduct of the reaction, out of the fuel cell so that it does not obstruct the cathode surface. The level of air flow required to transport water vapor will typically be varied as a function of the ambient temperature and relative humidity. Cool, relatively humid air is not as capable of extracting moisture from the fuel cell as hot, relatively dry air and, accordingly, greater air flow is required to extract the moisture when the air is relatively cool and humid. Another byproduct of the fuel cell reaction is heat, which much be removed from the fuel cell in order to prevent thermal runaway as well as the associated material set failure and, in the case of proton exchange membrane (“PEM”) fuel cells, membrane dry out. In addition to using the cathode air to remove heat from the fuel cell, many fuel cell systems include heat exchangers. Heat exchangers draw heat away from the fuel cell and air flow is used to remove heat from the heat exchanger. Relatively hot air is not as capable of removing heat from the heat exchanger as relatively cool air and, accordingly, greater air flow over the fuel cell and heat exchanger is required to cool the fuel cell when the air is relatively hot.
The inventor herein has determined that there are ambient conditions which cause the operating requirements to be in conflict with one another. When the air is hot and dry, for example, cooling the fuel cell will require a relatively high air flow rate because the air is hot. However, because the air is also dry, the high air flow rate can result in too much moisture being removed from the PEM if only cathode air is used to cool the fuel cell. This can, ultimately, lead to cell failure. One possible answer to the problems associated with conflicting operating requirements is to provide a pair of fans, one of which directs air to the cathode and one of which directs air to a heat exchanger. The fans are powered by the fuel cell during steady state operation and reduce the net power available from the fuel cell. The inventor herein has determined that it is undesirable to have two fans drawing power from the fuel cell because it unduly reduces the amount of power available for other uses.